For a wavelength division multiplex (hereinafter referred to as WDM) system, a wavelength selective switch that changes a path of light on a wavelength basis is used.
FIG. 1A is a diagram illustrating an example of the configuration of a wavelength selective switch (WSS) 100 that includes micro electro mechanical systems (MEMS) mirrors. As illustrated in a side view of FIG. 1A, multiple input ports and multiple output ports are provided on the input and output side of the wavelength selective switch 100 and arranged in a single row in a direction perpendicular to the paper sheet of FIG. 1A. First optical systems 1011 to 101n are provided for the ports, respectively, and each include a lens and the like. WDM signal light (having wavelengths of λ1 to λm) that is incident on the input ports is output from ends of optical fibers, for example. Then, the WDM signal light is collimated by the first optical systems 101 (collimators, for example) (refer to a top view of FIG. 1). The collimated light is demultiplexed into light with a number m of wavelengths by a wavelength demultiplexer 102 such as a diffraction grating. Then, a second optical system 103 (collecting lens, for example) collects the light and causes the light to be incident on a path controller 105 for controlling and changing a path of an optical signal. The path controller 105 includes an MEMS array 104A that has a number m of MEMS mirrors 104, for example.
As illustrated in FIG. 2A, the MEMS mirrors 104 may at least rotate around an X axis so that angels of the MEMS mirrors 104 are changed. Thus, light that has an interested wavelength can be coupled from an arbitrary one of the input ports to an arbitrary one of the output ports by rotating an interested mirror around the X axis at an angle θ. In addition, the light can be coupled from the arbitrary input port to the arbitrary output port at an arbitrary attenuation rate by rotating the interested mirror around the X axis or a Y axis. As illustrated in a graph of FIG. 2B, the attenuation rate of the MEMS mirror 104 may be controlled by adjusting the angle of the MEMS mirror 104. In FIG. 2B, the abscissa of the graph indicates a value (μm) of (F×θ), where F is a focal length of the second optical system 103 and θ is the angle of the MEMS mirror 104. In FIG. 2B, the ordinate of the graph indicates the attenuation rate (dB).
The light is reflected from the path controller 105 and collimated by the second optical system 103. The collimated light is introduced by the wavelength demultiplexer 102 into an arbitrary output port that has been selected based on the rotational angle of the MEMS mirror 104 around the X axis for each of the wavelengths. Then, the light is coupled to an interested optical fiber by the first optical system 101 at an attenuation rate determined based on the rotational angle of the MEMS mirror 104 around the X axis or the Y axis. When the WSS 100 has only a single input port and a single output port, only an attenuation rate is set for each of the wavelengths by the MEMS mirror array 104A. When the WSS 100 has a single input port and a plurality of output ports, the WSS 100 operates as a DROP type WSS that causes light with an arbitrary wavelength to be coupled from the single input port (common port) to any of the output ports. When the WSS 100 has a plurality of input ports and a single output port, the WSS 100 operates as an ADD type WSS that causes light with an arbitrary wavelength to be coupled from any of the input ports to the single output port (common port). The WSS 100 may have a plurality of input ports, a plurality of output ports and a wavelength demultiplexer that is shared.
FIG. 1B illustrates an ADD type WSS that has a number N of input ports and a single output port. Light that is input to the number N of the input ports is collimated by a collimator (first optical system) 101. Then, the light is demultiplexed into light with wavelengths λ1 to λm by a diffraction grating (wavelength demultiplexer) 102. The demultiplexed light is collected by a second optical system 103. After that, the light is incident on an MEMS array 104A. Mirrors 104 of the MEMS array 104A each cause light with an interested wavelength to be coupled from an arbitrarily selected one of the input ports to the output port. In addition, angles of the mirrors are controlled so that the mirrors have arbitrary attenuation rates.
In order to cause a diffraction grating element (to be used to select a wavelength) to offset or reduce shifting (owing to a variation in a temperature) of an angle of diffraction of light with a selected center wavelength, a configuration in which a diffraction grating is relatively rotated when the temperature increases has been proposed.
A related technique is disclosed, for example, in Japanese Laid-open Patent Publication No. H06-331850.
In an optical communication network, a signal that is transmitted may pass through wavelength selective switches (WSSs) of multiple nodes. In order to prevent a waveform of the signal from being degraded, the WSSs preferably each have a transmission band property that enables light having a band that is nearly equal to or wider than a band of the signal (signal light) to pass through the WSS. Especially, when the transmission rate is a high rate of 40 GHz, 100 GHz or the like (which has been used in recent years), spreading (caused by a modulation) of a wavelength spectrum of the signal light is large, the signal light is easily affected by the transmission band properties of the WSSs.
Referring to FIG. 3, in the following description, an entire transmission bandwidth that includes a transmission band of all channels (chs) is called a “pass band”; and a value that is obtained by doubling a narrower one of a long wavelength side band included in the pass band and a short wavelength side band included in the pass band is called a “clear pass band”. In this case, a boundary between the long wavelength side band and the short wavelength side band is determined using, as a reference, an ITU grid wavelength that is a wavelength determined by International Telecommunication Union Telecommunication Standardization Sector (ITU-T). In addition, a deviation of the center wavelength of the pass band from the ITU grid wavelength is called an “Off-ITU amount”. The “Off-ITU amount” means a “deviation of a wavelength”, which is described in this specification and claims. The Off-ITU amount is expressed using a frequency as a reference. When signal light is shifted toward a higher frequency, the Off-ITU amount is expressed using a positive sign. When the signal light is shifted toward a lower frequency, the Off-ITU amount is expressed using a negative sign.
When a signal is transmitted at a high rate, a clear pass band is preferably wide. Specifically, a wide clear pass band is requested, and an absolute value of the Off-ITU amount needs to be small. The WSS is generally achieved by a spatial optical system. Thus, the Off-ITU amount is determined by the accuracy of alignment of elements that form the spatial optical system. Specifically, when light that has a narrow spectrum and a wavelength equal to the ITU grid wavelength is to be incident in the configuration illustrated in FIG. 1A, the entire optical system needs to be aligned so that the light is incident on the center of an MEMS mirror 104, for example. For example, when the entire MEMS array 104A is shifted in a channel (ch) direction or a wavelength direction, Off-ITU amounts occur for all the channels and the clear pass band is narrowed.
However, it is very difficult to mechanically align the entire optical system. In general, after the entire optical system is mechanically adjusted to a certain extent, gas (such as He or Ar) that has a different refraction index is injected while Off-ITU amounts are monitored. A proportion of a component in internal gas or pressure of the gas is changed so that a refraction index is changed, and whereby the Off-ITU amounts are adjusted to smaller amounts. In this manner, the final fine adjustment is performed in the process of adjusting the gas so as to prevent the clear pass band from being narrowed.
In the aforementioned method, however, the adjustment is insufficient, and the Off-ITU amounts remain. Even when the adjustment is performed, the Off-ITU amounts have temperature characteristics in fact as illustrated in FIG. 4, and the clear pass band is narrowed. The cause of the temperature characteristics is not clear. The present inventor has found that when a temperature changes, the Off-ITU amounts vary by a nearly constant positive amount or a nearly constant negative amount regardless of the channels. Specifically, the inventor has found that the Off-ITU amounts vary depending on the temperature and the variations are nearly equal to each other regardless of the channels (dependencies of the Off-ITU amounts on the channels are low). Thus, it is considered that one of causes of the variations in the Off-ITU amounts is an alignment shifted owing to a mechanical deformation (or distortion) caused by a change in the temperature. The variations (in the Off-ITU amounts) that do not depend on the channels and are nearly equal to each other are merely called “Off-ITU variations”.